Conventionally, in an explosion-proof device such as a pressure transmitter, a hermetically sealed container serves as an explosion-proof container, a magnetic sensor is arranged in the explosion-proof container, and a switch structure that turns ON/OFF the magnetic sensor from the outside of the explosion-proof container is used (for example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 3-500939 (Japanese Patent No. 2668571).
FIG. 6 shows a primary portion of a conventional switch structure used in an explosion-proof device. In the drawing, reference sign 10 denotes an explosion-proof container, 20 denotes a magnetic sensor arranged in the explosion-proof container 10, and 30 denotes a magnet generating a magnetic field. A container wall 10a that separates the inside of the explosion-proof container 10 from the outside is a non-magnetic body. Also, the magnet 30 is provided outside the explosion-proof container 10 movably back and forth with respect to the magnetic sensor 20. Although not shown, the explosion-proof container 10 houses an electric circuit and an electric part to be protected.
With this switch structure, if the magnet 30 located outside the container wall 10a of the explosion-proof container 10 is moved close to the magnetic sensor 20, the magnetic field of the magnet 30 acts on the magnetic sensor 20 through the container wall 10a, and the magnetic sensor 20 is turned ON. That is, the magnetic sensor 20 senses the magnetism from the magnet 30 acting through the container wall 10a, and outputs a magnetism sensing signal. If the magnet 30 is moved far from the magnetic sensor 20, the magnetic sensor 20 no longer senses the magnetism from the magnet 30, and the magnetic sensor 20 is turned OFF.
The switch structure using the magnetic sensor 20 and the magnet 30 allows the operation of the electric circuit housed in the explosion-proof container 10 to be switched and the various settings of the electric circuit to be made from the outside while keeping the explosion-proof performance of the inside of the explosion-proof container 10. As shown in FIG. 7, this switch structure typically has a configuration in which the magnetic sensor 20 and the magnet 30 make a pair, the pair serves as a single magnetic switch 40, and a plurality of the magnetic switches 40 are arranged in parallel.
In an example shown in FIG. 7, magnetic sensors 20-1 to 20-4 are provided in parallel in the explosion-proof container 10, magnets 30-1 to 30-4 are provided outside the explosion-proof container 10 movably back and forth with respect to the magnetic sensors 20-1 to 20-4, and the magnetic sensors 20-1 to 20-4 and the magnets 30-1 to 30-4 configure magnetic switches 40-1 to 40-4. The container wall 10a being the non-magnetic body is located between the magnetic sensors 20-1 to 20-4 and the magnets 30-1 to 30-4.
In the switch structure with the plurality of magnetic switches 40 arranged in parallel, a distance L between adjacent two of the magnetic switches 40 is determined as a distance to prevent one magnet 30 from being influenced by the magnetic field of another magnet 30 so that each of the magnetic switches 40 can be independently turned ON/OFF. That is, since the container wall 10a is the non-magnetic body, the magnetic field of each magnet 30 is spread in a wide range. Hence, the distance L between adjacent two of the magnetic switches 40 is sufficiently determined to prevent the magnetic field of the magnet 30 from acting on the other magnetic sensors 20.